I am reading Marijn Haverbeke's excellent book, "Eloquent JavaScript". https://eloquentjavascript.net/
I do not understand this example on closures where number is not defined, yet there is no error.
Is number a function or a parameter?
There is nothing to indicate that number is passed in as parameter the second time.
function multiplier(factor) {
return number => number * factor;
}
let twice = multiplier(2);
console.log(twice(5));
// → 10
Understanding how closures work is hard enough but when the example you're looking at arbitrarily mixes a function declaration with an arrow function - as this one does - if you don't understand how arrow functions work it makes it more difficult to understand.
Here's a slightly easier example that doesn't use an arrow function to show what's going on.
// `multipler` takes a factor as an argument
function multiplier(factor) {
// It returns a function that - when it's called -
// accepts a number
return function (number) {
// And the return from that function
// is the factor * number
return number * factor;
}
}
// So we call `multipler` with a factor and assign the
// function it returns to our `twice` variable
let twice = multiplier(2);
// We can then call the function assigned
// to `twice` with a number, and the resulting
// return from that function will be factor * number
console.log(twice(5));In terms of the example using that arrow function:
// We pass in a factor to the `multipler`
function multiplier(factor) {
// We return a function that accepts a number
// and returns factor * number
// (I've added parentheses around the number
// parameter to clearly show it)
return (number) => number * factor;
}
// So we call `multipler` with a factor and assign the
// function it returns to our `twice` variable
let twice = multiplier(2);
// We can then call the function assigned
// to `twice` with a number, and the resulting
// return from that function will be factor * number
console.log(twice(5));